N-Heterocyclic Olefins as Organocatalysts for Polymerization: Preparation of Well-Defined Poly(propylene oxide)

Article abstract of DOI:10.1002/anie.201504175

The metal-free polymerization of propylene oxide (PO) using a special class of alkene - N-heterocyclic olefins (NHOs) - as catalysts is described. Manipulation of the chemical structure of the NHO organocatalyst allows for the preparation of the poly(propylene oxide) in high yields with high turnover (TON>2000), which renders this the most active metal-free system for the polymerization of PO reported to date. The resulting polyether displays predictable end groups, molar mass, and a low dispersity (D strok sign_M<1.09). NHOs with an unsaturated backbone are essential for polymerization to occur, while substitution at the exocyclic carbon atom has an impact on the reaction pathway and ensures the suppression of side reactions. One carbon makes a difference: The efficient and controlled formation of poly(propylene oxide) (PPO) at a very low catalyst loading of N-heterocyclic olefins showcases the use of this group of highly polarized alkenes as catalysts for organopolymerization. A strong structure-activity relationship is found, which is fundamentally different from the reactivity of N-heterocyclic carbenes (TON=turnover number, PDI=polydispersity index).

Full text of DOI:10.1002/anie.201504175

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International Edition: DOI: 10.1002/anie.201504175
German Edition: DOI: 10.1002/ange.201504175
Ring-Opening Polymerization
N-Heterocyclic Olefins as Organocatalysts for Polymerization:
Preparation of Well-Defined Poly(propylene oxide)**
Stefan Naumann,* Anthony W. Thomas, and Andrew P. Dove*
Abstract: The metal-free polymerization of propylene oxide
(PO) using a special class of alkene—N-heterocyclic olefins
(NHOs)—as catalysts is described. Manipulation of the
chemical structure of the NHO organocatalyst allows for the
preparation of the poly(propylene oxide) in high yields with
high turnover (TON > 2000), which renders this the most active
metal-free system for the polymerization of PO reported to
date. The resulting polyether displays predictable end groups,
molar mass, and a low dispersity (“M < 1.09). NHOs with an
unsaturated backbone are essential for polymerization to
occur, while substitution at the exocyclic carbon atom has an
impact on the reaction pathway and ensures the suppression of
Scheme 1. a) Mesomeric structures for NHOs; b) Fürstner’s NHO-
side reactions.
rhodium complex, and c) transfer to monomer as a side reaction in
PO polymerization. M+ =organic or inorganic counterion.
N
-Heterocyclic olefins (NHOs), cyclic derivatives of ketene
aminals (ene-1,1-diamines), comprise a group of highly
polarized alkenes.[1] These compounds bear considerable
electron density on their exocyclic carbon atom, which can
be formally denoted as charge separation (Scheme 1a). This
unusual characteristic provides NHOs with remarkable
properties, some of which have been exploited for the
synthesis of NHO–metal complexes,[2] the formation of
NHO–CO2 adducts,[3] or in Diels–Alder reactions.[4] Remark-
ably, Fürstner et al. found that even a very simple NHO (1,3-
dimethyl-2-methyleneimidazoline Scheme 1b) confers more
electron density onto the metal center (see A) than typical N-
heterocyclic carbenes (NHCs).[5] Moreover, coordination to
the metal center was established to be end-on, in contrast to
the commonly encountered side-on coordination of olefins,
a clear testament to the strong polarization of the double
bond. Furthermore, NHOs were also recently applied in
Lewis pairs with aluminum-based co-catalysts for the poly-
merization of acrylates.[6]
The structural versatility of NHOs makes them exciting
species that need to be explored more fully, the more so since
they are closely related to so-called deoxy-Breslow inter-
mediates, which are proposed to feature a prominent role in
NHC-based organocatalysis.[7] Importantly, there are many
ways to actively tune the degree of the double-bond polar-
ization, which opens the possibility to design the reactivity of
the NHO. This can be achieved simply by manipulation of the
heterocycle. Charge separation is expected to be significantly
favored with an unsaturated five-membered backbone, as the
cyclic moiety can aromatize; this effectively “captures” the
positive charge and maximizes the electron density on the
exocyclic carbon atom. Likewise, positions R1–R3 can easily
be varied by making use of well-established synthetic routes
to form N-heterocycles. Although this structural diversity has
so far only been explored to a limited degree, it potentially
couples NHC-like adaptability with carbanionic reactivity.
These characteristics suggest that NHOs will be ideally suited
for application in a field where high nucleophilicity and
basicity are essential.
Despite many recent advances in organocatalyzed poly-
merization, challenges remain to discover methods for the
efficient polymerization of several important monomers.
Propylene oxide, oligomers or polymers of which are mainly
used in industry as long-chain polyether-polyol components
for polyurethane formation,[8] is a prime example, as it has
proven difficult to polymerize in the absence of metal
activation. Organocatalysts alone typically display long
reaction times, low turnover numbers (TON), or side
reactions such as transfer to monomer (Scheme 1c), which
severely limit the accessible molecular weights and end-group
fidelity.[9] Phosphazene bases have been successfully applied
to polymerize ethylene oxide,[10,11] but were reported to
generate considerable levels of transfer to monomer when PO
[*] Dr. S. Naumann, A. W. Thomas, Prof. A. P. Dove
Department of Chemistry, University of Warwick
Materials and Analytical Science Building, CV4 7AL Coventry (UK)
E-mail: s.naumann@warwick.ac.uk
a.p.dove@warwick.ac.uk
[**] S.N. gratefully acknowledges his DFG research stipend (NA 1206/
1). The Royal Society is also acknowledged for the award of an
Industry Fellowship to A.P.D. The University of Warwick and EPSRC
are also acknolwedged for funding.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504175.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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was used.[12] Perhaps more relevant to these investigations,
Taton, Gnanou, and co-workers described an elegant, solvent-
free process catalyzed by NHCs, which enabled the prepara-
tion of poly(propylene oxide) (PPO) with well-defined end
groups and good control over the molecular weight.[13]
However, yields were limited to 30–40% at long reaction
times (3 days) at 508C. NHC–CO2 adducts have also been
used for the oligomerization of PO, although high catalyst
loadings and high temperature are required.[14] Herein, we
present the first application of NHOs as organic catalysts to
overcome the difficulties in the metal-free synthesis of PPO.
NHOs 1–3 (Scheme 2a) were prepared to determine the
influence of the ring architecture on the reactivity. By using
a convenient procedure to generate the target molecules from
Table 1: Bulk polymerization of PO at 508C using NHOs 1–4.
Entry NHO Time
[h]
NHO/BnOH/ Conversion
PO
Mn
“
M
[%][a]
[gmolÀ1 [b]
]
1
2
3
4
5
6
7
8
1
2
3
4
3
4
3
4
18.5
18.5
18.5
18.5
18.5
18.5
68
1:10:1000
1:10:1000
1:10:1000
1:10:1000
1:0:1000
1:0:1000
1:10:1000
1:10:1000
0
0
–
–
–
–
1.04
1.06
2.47
–
57
43
<5
0
96
88
3500
3500
1800[c]
–
5600
6700
1.06
1.04
68
[a] Calculated from 1H NMR spectra. [b] Determined by GPC analysis
(CHCl3, polystyrene standards). [c] GPC chromatogram multimodal.
siloxanes,[19] a mechanistic duality that involves both “basic”
and “nucleophilic” mechanisms has been proposed.[7c,20,21] It is
reasonable to assume that two different mechanistic pathways
are also possible for NHO-catalyzed polymerization
(Scheme 3): i) deprotonation of the initiator to induce
a more classical anionic polymerization of PO, with the
NHO as a non-innocent counterion that interacts with the
propagating chain end (equivalent to the “basic” mechanism
in NHC-catalyzed processes) and ii) nucleophilic attack of the
NHO on the monomer with subsequent zwitterionic poly-
merization (equivalent to the “nucleophilic” mechanism in
NHC-catalyzed processes). Liberation of the catalyst from
the latter, proposed zwitterionic state, by nucleophilic sub-
stitution is strongly disfavored for NHCs, as shown by recent
DFT calculations.[14] Such an elimination is even less likely for
NHOs on account of the difference made by the additional
carbon atom in the zwitterionic structure, hence trapping the
NHO and preventing interconversion of the two propagating
species, thus resulting in a multimodal molecular-weight
distribution. We therefore reasoned that NHO 3 should also
yield PPO in the absence of BnOH. Indeed, the subsequent
bulk ROP of PO in the presence of only NHO 3 (Table 1,
entry 5) resulted in isolation of a polymer, although in low
yield (< 5%, Figure S4). Analysis of the resultant polymer by
GPC revealed a multimodal distribution with a high molec-
ular weight (number-average molecular weight, Mn, up to
11000 gmolÀ1).
To produce well-defined PPO, we sought to block the
supposedly zwitterionic pathway by increasing the steric
congestion of the catalytically active site. Hence, NHO 4 was
synthesized, which bears two methyl groups on the exocyclic
carbon atom. In addition to increased steric demands, this
modification was proposed to enhance the basicity of the
NHO on account of the generation of a latent tertiary
carbanion (NHO 3 would generate a primary carbanion), and
thus favor the anionic over the zwitterionic mechanism.
Gratifyingly, under the same conditions outlined above, NHO
4 yielded PPO with low dispersity and a monomodal molec-
ular weight distribution (Table 1, entry 4; Figure S3).
Although this was coupled with a slightly decreased activity
compared to NHO 3, the molecular weight of the prepared
polymer was fully predictable from the monomer/initator
ratio as a consequence of the elimination of the side reactions
(Figure S5). Importantly, NHO 4 did not yield any polymer
when treated with PO in the absence of alcohol (Table 1,
Scheme 2. a) Catalysts prepared for this study and b) generalized
synthetic procedure.
their precursor salts[3,15] by deprotonation with KH,[1b, 2e,5,16]
(Scheme S1), the NHOs were obtained after filtration and
evaporation of the solvent, and could be used without further
purification. Notably, NMR spectroscopic analysis revealed
that the signals for the olefinic CH2 protons appear strongly
shifted towards high field (d = 3.29–2.84 ppm, Figure S2), thus
mirroring the increased electron density. NHO 3 displayed
the strongest shift, in accordance with literature data,[1,2]
which can be attributed to the stronger contribution of the
charge-separated mesomeric state that is a consequence of
aromatization. The ring-opening polymerization (ROP) of
PO was investigated in the bulk phase at 508C in the presence
of benzyl alcohol (BnOH) as an initiator at only 0.1% catalyst
loading (NHO/BnOH/PO = 1:10:1000). The initial results
immediately revealed the impact of the N-heterocyclic ring
system. While NHOs 1 and 2 did not yield any polymer, NHO
3 generated PPO with 57% monomer conversion after 18.5 h,
thus rendering this NHO more than four times more active
than the benchmark NHC-based setup[13] (Table 1, entry 3).
Moreover, when the reaction time was extended, near
quantitative yields (96%) were achieved (Table 1, entry 7).
However, although analysis by gel-permeation chromatogra-
phy (GPC) showed a very well-defined main peak (“M <
1.07), a small high-molecular-weight impurity was also
observed (Figure S3). Although minor, this observation
suggested the presence of at least two different propagating
species.
While NHC-catalyzed polymerization has been extended
to the ROP of PO,[13] ethylene oxide,[17] lactones,[18] and
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Scheme 3. Major (anionic) and minor (zwitterionic) mechanisms proposed for the polymerization of PO using 3. Box: Zwitterion derived from NHC.
entry 6), thereby revealing a sharp contrast to typical NHC
reactivity.[13] MALDI-ToF MS analysis of the PPO produced
using NHO 4 underlined the well-defined nature of the
NHO 4 resulted in the appearance of a single broad resonance
at d = 13.17 ppm, which most likely represents a significantly
deshielded alcoholic proton.[22] In contrast, a broad signal is
observed at d = 4.65 ppm in the presence of NHO 3. Most
likely, this series of spectra show BnOH–NHO complexes in
different stages of the deprotonation process: the stronger
base, 4, is able to abstract the proton of the alcohol to a higher
degree than 3, the weaker base. Consistently, the methylene
unit Ar-CH2-OH is shifted stepwise towards low field (d =
4.44, 4.67, 5.05, and 5.19 ppm for NHOs 1–4, respectively) and
likewise the convoluted aromatic region of BnOH becomes
increasingly differentiated. Both observations are again in
line with increasing deprotonation, which will increase the
negative charge on the oxygen atom (and therefore the
shielding of the adjacent methylene moiety) and also
strengthen the inductive effect on the aromatic ring. This
outcome strongly emphasizes the difference in the reactivity
of the studied NHOs. Since NHOs 1 and 2 do not yield PPO,
but NHOs 3 and 4 do, these interactions seem critical to
defining the reactivity that has to be surpassed to induce
polymerization.
Once it had been established that catalyst 4 was able to
produce highly defined and monomodal PPO, further inves-
tigations of the key properties of the polymerization of PO
catalyzed by this NHO were undertaken. Identical but
independent polymerizations were stopped after different
reaction times, to correlate the conversion and molecular
weight. Notably, this resulted in a perfectly linear relationship
(Figure 2) over the range 1700–6700 gmolÀ1, while maintain-
ing low dispersity (“M < 1.09). Additionally, with the pro-
posed role of the NHO as the catalyst and BnOH as the
initiator, a change in the ratios was anticipated to have
distinct consequences on the molecular weight of the
resultant PPO. Although a higher loading of NHO 4 at
constant BnOH/PO resulted in a more rapid polymerization,
the molecular weight remained within the expected range
(Table 2, entry 1; compare Table 1, entry 4 and Figure S5). In
a complementary manner, doubling the amount of BnOH (to
target a degree of polymerization (DP) of 50) halved the
resulting molecular weight, while a target DP = 200 yielded
PPO with a molecular weight greater than 10000 gmolÀ1
(Table 2, entries 2 and 4). When the target DP was increased
further to 300, the monomer conversion dropped significantly
polymerization (Figure 1).
A
single distribution was
observed, with the major signal in the spectrum consistent
with that calculated for a sodium-charged PPO initiated by
benzyl alcohol. As supported by NMR spectroscopic analysis
of the polymer, no allyl-terminated polymer chains were
observed which indicates an absence of hydrogen abstraction
as a relevant side reaction (Scheme 1).[9] Similar MALDI-ToF
MS analysis of the PPO generated by NHO 3 additionally
showed the expected high-molecular-weight impurity (Fig-
ure S6). These observations further corroborate the proposed
polymerization mechanism.
Clearly, the interaction of NHOs 1–4 with hydroxy groups
is crucial for the success of the polymerization. To investigate
1
these interactions further, a series of H NMR spectroscopy
experiments was conducted (C6D6, BnOH/NHO = 1:1, ambi-
ent temperature; Figures S7 and S8). The addition of NHO
1 to BnOH resulted in the disappearance of both the olefinic
and -OH signals, which indicates some degree of interaction;
the other signals showed little or no shift. However, this
gradually changed when going from NHO 2 and 3 to NHO 4.
Figure 1. a) MALDI-ToF mass spectrum of PPO prepared by 4 (Table 1,
entry 4), b) expansion, and c) calculated mass of sodium-charged PPO
initiated by BnOH.
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lining the robustness of the catalyst and the absence of
relevant poisoning by impurities. In both cases, transfer to
monomer was insignificant (Figure S11).
In conclusion, we have reported the first example of an
NHO-catalyzed organopolymerization. Through manipula-
tion of the NHO structure we have shown that the nature of
the heterocyclic ring is a key factor in determining the activity
in the polymerization of PO. Imidazolium-based catalysts
show a high performance, while their saturated five- and six-
membered counterparts do not polymerize PO at all.
Furthermore, it was demonstrated that the exocyclic carbon
atom is a decisive tuning site of the catalysts, and can be used
to suppress undesired side reactions. Increased steric con-
gestion and basicity, as present in NHO 4, enabled the
synthesis of well-defined PPO in a highly controlled manner,
while at the same time, to the best of our knowledge, this
NHO emerged as the most active organocatalyst for PO
polymerization reported to date. The distinct NHO carba-
nionic reactivity can be expected to allow access to further
highly challenging fields in organopolymerization, especially
since the catalysts can be readily tailored, as shown in this
study.
Figure 2. Monomer conversion versus molecular weight (Mn, GPC).
4/BnOH/PO=1:10:1000, 508C, bulk.
Table 2: Bulk polymerization of PO at 508C using NHO 4.
Entry Time
[h]
NHO/BnOH/
PO
Conversion
[%][a]
Mn
“
M
[gmolÀ1 [b]
]
1
2
3
4
5
6
7
8
18.5
68
68
68
68
138
113
113
2:10:1000
1:20:1000
1:10:1000
1:5:1000
1:3.3:1000
1:3.3:1000
1:30:3000
1:100:10000
66
92
88
84
59
75
73
26
5100
3900
6700
10100
9300
12000
5800
2100
1.04
1.06
1.04
1.05
1.06
1.06
1.04
1.06
Keywords: N-heterocyclic olefins · organocatalysis ·
poly(propylene oxide) · ring-opening polymerization
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9550–9554
Angew. Chem. 2015, 127, 9686–9690
[a] Calculated from 1H NMR spectra. [b] Determined by GPC analysis
(CHCl3, polystyrene standards).
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(Table 2, entry 5) and remained below the level of the lower
DP polymerizations even when the reaction time was
extended (Table 2, entry 6). Nonetheless, a molecular weight
of 12000 gmolÀ1 was achieved in this way, which is atypically
high for PPO derived by organocatalysis. These results
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Received: May 7, 2015
Published online: July 1, 2015
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